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read(2)								       read(2)

Name
       read, readv - read from a file

Syntax
       cc = read(d, buf, nbytes)
       int cc, d;
       char *buf;
       int nbytes;

       #include <sys/types.h>
       #include <sys/uio.h>

       cc = readv(d, iov, iovcnt)
       int cc, d;
       struct iovec *iov;
       int iovcnt;

Arguments
       d	 File descriptor.

       buf	 Character pointer where information is stored.

       nbytes	 Integer that tells you how many bytes to read.

       iov	 Pointer to an iovec structure.

       iovcnt	 The number of iovec structures to be processed.

Description
       The  system call attempts to read nbytes of data from the object refer‐
       enced by the descriptor d into the buffer pointed to by buf.  The  sys‐
       tem call performs the same action, but scatters the input data into the
       iovcnt buffers specified by the members of the iovec  following	array:
       iov[0], iov[1], ..., iov[iovcnt-1].

       For the iovec structure is defined as follows:

       struct iovec {
	    caddr_t   iov_base;
	    int	 iov_len;
       };

       Each  iovec  entry  specifies the base address and length of an area in
       memory where data should be placed.  The system call fills an area com‐
       pletely before proceeding to the next area.

       On  objects that are capable of seeking, the starts at a position given
       by the pointer associated with d.   See	for  more  information.	  Upon
       return  from the pointer is incremented by the number of bytes actually
       read.

       Objects that are not capable of seeking always read  from  the  current
       position.   The	value of the pointer associated with such an object is
       undefined.

       When attempting to read from an empty pipe (or FIFO):

       ·    If no process has the pipe open for writing, returns zero to indi‐
	    cate end-of-file.

       ·    If	some  process  has  the	 pipe open for writing and O_NDELAY or
	    O_NONBLOCK is set, returns a -1, errno is  to  [EWOULDBLOCK].   If
	    some process has the pipe open for writing and O_NDELAY and O_NON‐
	    BLOCK are clear, blocks until data	is  written  or	 the  pipe  is
	    closed by all processes that opened the pipe for writing.

       Upon  successful	 completion,  and  return the number of bytes actually
       read and placed in the buffer.  The system reads the  number  of	 bytes
       requested if the descriptor references a file which has that many bytes
       left before the end-of-file; this is not true in any other instance.

       Unless the SV_INTERRUPT bit has been set for a signal, the system calls
       are  automatically  restarted  when  a  process receives a signal while
       waiting for input. See also

Return Values
       If the returned value is 0, then end-of-file has been reached.

       If the read is  successful,  the	 number	 of  bytes  actually  read  is
       returned.  Otherwise, a -1 is returned and the global variable errno is
       set to indicate the error.

Diagnostics
       The and system calls fail if one or more of the following are true:

       [EBADF]	      The d argument is not a valid file or socket  descriptor
		      open for reading.

       [EFAULT]	      The buf points outside the allocated address space.

       [EINTR]	      A	 read  from  a	slow device was interrupted before any
		      data arrived by the delivery of a signal.

       [EIO]	      An I/O error occurred while reading from the  file  sys‐
		      tem.

       [ESTALE]	      The  file	 handle	 given in the argument is invalid. The
		      file referred to by that file handle no longer exists or
		      has been revoked.

       [EWOULDBLOCK]  The  O_DELAY  or	O_NONBLOCK  flag  is  set for the file
		      descriptor and the process would be delayed in the  read
		      operation.

       In addition, may return one of the following errors:

       [EINVAL]	      The  iovcnt was less than or equal to 0, or greater than
		      16.

       [EINVAL]	      One of the iov_len values in the iov array was negative.

       [EINVAL]	      The sum of the iov_len values in	the  iov  array	 over‐
		      flowed a 32-bit integer.

       [EFAULT]	      Part  of	the iov points outside the process's allocated
		      address space.

       [ETIMEDOUT]    A	 connect  request  or  remote  file  operation	failed
		      because  the  connected  party  did  not respond after a
		      period of time determined by the	communications	proto‐
		      col.

Environment
       SYSTEM_FIVE

       When you use the System V environment, note the following:

       ·    If your program is compiled in this environment, a and system call
	    returns 0 if the file has been set up for non-blocking I/O and the
	    read would block.

       ·    In	this  environment, the parameter nbytes is of type int instead
	    of type unsigned.

       POSIX

       In the POSIX environment, [EAGAIN] is  returned	in  errno  instead  of
       [EWOULDBLOCK].

See Also
       dup(2), open(2), pipe(2), sigvec(2), socket(2), socketpair(2)

								       read(2)
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